Dinosaur-killing rock was a twin terror

ASTEROIDS 2, dinosaurs 0. The infamous space rock that slammed into Earth and helped wipe out the dinosaurs may have been a binary - two asteroids orbiting each other. This surprise conclusion emerges from a new look at the likelihood of binary craters forming on Earth, and could spell bad news for those hoping to protect our world from future catastrophic collisions.

Earth bears the scars of a few twin-asteroid impacts: the Clearwater Lakes near Hudson Bay in Canada, for instance, are craters that formed together about 290 million years ago. But just one in 50 craters on Earth comes in a pair. That is a puzzle because counts of the rocks zooming around near Earth suggest binary asteroids are far more common than that.

"It's been known for 15 years that about 15 per cent of near-Earth asteroids are binary," says Katarina Miljkovic at the Institute of Earth Physics in Paris, France. All else being equal, 15 per cent of Earth's impact sites should have been created by twin objects. Why does the actual proportion appear to be so much smaller?

Miljkovic and her colleagues have found an explanation. Their computer simulations of binary asteroids hitting Earth often result in a single crater (see images at right). This makes sense, given that a crater can be 10 times the diameter of the asteroid that made it. The crater near Chicxulub in Mexico - thought to be the result of an impact 65.5 million years ago that wiped out large dinosaurs - is about 180 kilometres wide. The culprit was an asteroid 7 to 10 kilometres wide, or so we had thought.

Only impacts involving two small, widely separated asteroids are guaranteed to form a distinct pair of craters, the simulations show (Earth and Planetary Science Letters, doi.org/kcx). However, such binaries are rare, explaining why Earth boasts so few of these pairs.

The work implies that binary asteroids hit Earth more often than the crater record appears to suggest - with implications for efforts to prevent future impacts (see "Do twin asteroids pose twice the risk?"). The simulations also suggest that it is possible to identify which of Earth's single craters had binary origins. These craters should be subtly asymmetrical, and that makes Chicxulub a strong candidate, says Miljkovic. "It is worth considering that it was formed by a binary asteroid."

Petr Pravec at the Academy of Sciences of the Czech Republic in Ondrejov agrees, saying that recent gravity surveys taken at the Chicxulub impact area support Miljkovic's conjecture. As for what the Chicxulub asteroids might have looked like, Miljkovic's simulations - coupled with the crater's size and shape - suggest that their combined diameter was equal to that of the single asteroid we previously imagined, and that they were up to 80 kilometres apart. "But these numbers are just guidelines," she says.

It has long been suspected that binary asteroids can generate single craters, says Jean-Luc Margot at the University of California, Los Angeles. "The new study puts this conjecture on solid analytical footing."

This article appeared in print under the headline "Dinosaur-killing space rock was a terrible twosome"

Do twin asteroids pose twice the risk?

If binary asteroids can form single craters, then Earth is more likely to be hit by a pair of objects in future than our planet's crater record would suggest. Could these double whammies be harder to spot or deflect than single asteroids?

The existential threat posed by asteroids has gained attention in recent years. Underlining the risk, one will skim Earth on 15 February. Several new efforts are afoot to scan the skies for asteroids, and we have a plethora of suggestions for how they might be deflected (New Scientist, 26 January, p 42). But can any of these schemes deflect a binary object?

"A nuclear explosion might be directed at the smaller body, blowing it away," says Alan Harris, a retired NASA researcher. "The recoil on the main asteroid might deflect it from a collision course."

But being able to deploy the right defence would depend on our ability to spot whether something heading our way is a binary. Don Yeomans of NASA's Near Earth Object Program thinks that won't be a problem for future asteroid-hunting spacecraft. "There is a slim chance that the autonomous navigation camera might be confused with two images in its field of view, but I should think these issues would be easily overcome," Yeomans says.

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